Quadrupole mass spectrometers are known. A portion of a prior art quadrupole mass spectrometer is shown in FIG. 1 of the drawings. A ground plate 10 supports an electron source filament 12 via two conductive posts 14. The electron source filament 12 carries a current of sufficient magnitude so that electrons having a negative charge are emitted by the electron source filament 12, i.e., the electron source filament 12 serves as a cathode. The electrons are accelerated electrostatically towards an ion source cage 16 by an electric field in the region between an electron repeller cage 18 and the ion source cage 16. Positive ions are produced inside the ion source cage 16 when the accelerated electrons strike neutral gas particles, in the form of atoms and molecules or a mixture thereof, within the cage along the path of the electrons. The positive ions within the ion source cage 16 are accelerated towards a quadrupole mass filter 20 by a focus plate 30. The quadrupole mass filter 20 includes a quadrilaterally symmetric parallel array of four rods 22. Prior to entering the interior of the quadrupole mass filter 20, the positive ions pass through an aperture 24 of a total pressure measurement plate 26 described in greater detail below.
To obtain an indication of the mass spectrum of the ions in the space defined by the ion source cage 16, a constant (DC) and superimposed sinusoidally modulated (RF) voltage is applied to the rods 22 of the quadrupole mass filter 20, and are scanned in tandem such that their ratio remains constant. More specifically, each diametrically opposite pair of rods are connected together. A signal (U+Vcos.omega.t), which includes a positive DC component (U) and a radio frequency (RF) component (Vcos.omega.t), is applied to one pair of rods, while a signal (-U -Vcos.omega.t), which includes a negative DC component (-U) and a radio frequency (RF) component (-Vcos.omega.t) opposite in phase (180.degree.) to the RF component of the first mentioned signal, is applied to other pair of rods. The DC and RF component signals are scanned such that their ratio of amplitudes, U/V, is kept constant. The fraction of the total ion current that exits the quadrupole mass filter 20 is partitioned according to the mass-to-charge ratio of each ion of the ion current. By scanning the RF voltage component from a low to a high value, a plurality of ions, each having a particular mass-to-charge ratio and arriving simultaneously at the entrance to the quadrupole mass filter 20, will arrive sequentially and ordered according to mass-to-charge ratio at the exit of the quadrupole mass filter 20. Generally, by scanning the RF voltage component from a low to a high value, ions having a relatively low mass-to-charge ratio will arrive at the end of the quadrupole mass filter 20 before ions having a relatively high mass-to-charge ratio. The ion current exiting the filter 20 is sensed by a detector (not shown), such as a Farraday cup.
In the prior art device described with respect to FIG. 1, the total pressure plate 26 is used to measure the total pressure of the gas within the device. Since the aperture 24 of the total pressure measurement plate 26 is smaller than the aperture 28 of the focus plate 30, a known fraction of the total ion current provided to the filter 20 is collected by the total pressure measurement plate 26. A current measurement device 29 connected to the pressure measurement plate 26 then provides a current signal as a function of the ion current collected by the total pressure measurement plate 26, and is therefore representative of the total ion current entering the quadrupole mass filter 20. Once the total ion current is known, the total pressure P.sub.T is obtained by multiplying the ion current by an empirically determined constant multiplicative factor. However, error can be introduced into this measurement of total pressure P.sub.T due to fringe field effects at the entrance of the quadrupole mass filter 20, such as "reflection" of ions back towards the plate 26 after they pass through the aperture 24. "Reflection" is a deflection of the original trajectory of the ions due to repulsive forces resulting from the fringe fields.
Accordingly, another known method of measuring total pressure P.sub.T has been developed that allows for the elimination of total pressure plate 26, thereby eliminating the problem created by the fringe field effect at the entrance of the quadrupole mass filter 20. In this prior art method, the DC voltage applied to the quadrupole mass filter 20 is set to 0 volts, and all of the ions that pass through the focus plate 30 will enter the quadrupole mass filter 20, and will be collected and measured at a detector (not shown in FIG. 1) as they exit the quadrupole mass filter 20. The ion current that is measured at the detector as the ions exit the quadrupole mass filter 20 represents the total ion current, from which can be calculated the total pressure P.sub.T. However, in practice, inaccuracies can arise due to lighter ions, such as hydrogen and helium gas ions, failing to arrive at the detector (not shown) at the exit of the quadrupole mass filter 20. Consequently, the value obtained for total pressure P.sub.T is significantly inaccurate.